posix: build and install modules correctly.

* configure.ac (POSIX_EXTRA_LDFLAGS): Rename from this...
(LIBRT): ...to this.
(LIBCRYPT): Set according to results of AC_SEARCH_LIBS.
* local.mk (lib_LTLIBRARIES): Moved from the C library tree...
(luaexec_LTLIBRARIES): ...to the Lua cpath tree.
(check_LTLIBRARIES): Move from here...
(EXTRA_LTLIBRARIES): ...to here.
(check_local): Ensure standalone modules build correctly...
(clean_local): ...and that file droppings are removed.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>